Hi have just serviced an old oil fired boulter classic 95/135 which has been out of use for some time - everything is up and running ok fires up etc etc but on replacing the front cover casing the boiler within a minute or so the flame fails, trys to relight and then lockout runs perfect without front cover - I know its an air problem but the boiler obviously worked at some time with the casing on, I opened up the air intake abit to see if it made a difference but to no avail ! the only thing I have noticed is that at the end of the low level flue outlet the sampling point plate has come away from the square housing that it sits in, I refitted it as it was laying in the bottom of the terminal guard! not too close to the outlet pipe of course but far enough in to serve its purpose. I'm not sure what to do here as I'm a bit flummoxed was putting the plate back the problem or am I missing something more basic?